Issue 66

M. Sánchez et alii, Frattura ed Integrità Strutturale, 66 (2023) 322-338; DOI: 10.3221/IGF-ESIS.66.20

M ATERIALS AND METHODS

T

he mechanical properties of FFF printed materials have gained significant interest due to widespread applications. In this study, we focus on PLA and PLA-Gr, the latter being graphene-additivated (1 wt.%) PLA. A previous work of the authors has already examined the basic tensile and fracture properties of these two materials [15]. Here, the investigation was extended by analyzing the behavior of structural plates containing different types of notches. The PLA and PLA-Gr plates were manufactured using an FFF-based Prusa i3 printer with a raster orientation of 45/-45, employing the same specific printing parameters reported in [15] for the tensile and SENB fracture specimens used in the basic characterization. The nozzle diameter was 0.4 mm, and the nozzle temperature was set at 200 ºC, while the bed temperature was maintained at 75 ºC. The printing rate was 30 mm/s, and the infill level was set at 100%. Layer height was maintained at 0.3 mm. The tensile tests reported in [15] followed ASTM D638 [16], whereas fracture tests on both cracked and notched specimens followed ASTM D6068 [17], as reported in [18]. The main mechanical properties derived from the tensile specimens and the cracked fracture specimens are gathered in Tab. 1 [15,18], while Fig. 1 shows examples of the obtained stress-strain curves revealing how PLA-Gr material develops a stress-strain curve which is much closer to linear-elastic conditions than that developed by PLA. Material Raster orientation E (MPa) σ y (MPa) σ u (MPa) ɛ u (%) K mat (MPa·m 0.5 ) PLA 45/-45 2751 ± 406 35.3 ± 4.6 41.1 ± 5.7 2.6 ± 0.2 4.91 ± 0.25 PLA-Gr 45/-45 3972 ± 260 47.5 ± 1.4 49.0 ± 2.8 1.5 ± 0.2 7.20 ± 0.31 Table 1: Mechanical properties for FFF printed PLA and PLA-Gr. E: Young’s modulus; σ y : yield stress; σ u : ultimate tensile strength; ɛ u : strain under maximum load; K mat : fracture toughness.

Figure 1: Tensile curves obtained for PLA and PLA-Gr.

A total of 78 plates were tested in this study, encompassing various notch configurations. The plates were classified as U notched and V-notched, and their geometries and dimensions varied as shown in the scheme of Fig. 2. Here it is important to mention that notches were machined instead of being printed, with the aim of achieving superior finishing and avoid undesirable filament orientation effects during the fracture process [19]. Tabs. 2 and 3 gather the main dimensions (measured values) of the specimens composing the experimental campaign performed in this work. For U-notched plates, the specimens and nominal dimensions were as follows:

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